In an RF receiver, such as a radio receiver for cellular communications, the incoming RF frequency must be down-converted to an intermediate frequency by mixing it with a local oscillator frequency.
In conventional direct zero-IF converters problems traditionally arise due to RF-LO, LO-RF, and LO-antenna leakage, DC offsets that arise due to mismatching of the devices used in a direct conversion RF, and the variable DC offsets that occur due to the direct down-conversion process itself.
In the prior art, these problems have been addressed by placing a metal shield on the direct conversion receiver to isolate it from the antenna. This reduces the amount of leakage from the local oscillator to the antenna.
The RF-LO and LO-RF leakage was solved by developing mixer circuitry that isolates the RF portion from the LO portion.
The DC offsets were reduced by using matched devices. This was accomplished by placing devices fairly close together on an IC.
Fairly complicated adaptive DC removal techniques have also been used, and low -IF instead of true Zero-IF has been used.